U.S. patent application number 11/447333 was filed with the patent office on 2007-12-06 for print head with reduced bonding stress and method.
Invention is credited to Gil Fisher, Haggai Karlinski, Roi Nathan, Ilan Weiss.
Application Number | 20070279455 11/447333 |
Document ID | / |
Family ID | 38692366 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070279455 |
Kind Code |
A1 |
Karlinski; Haggai ; et
al. |
December 6, 2007 |
Print head with reduced bonding stress and method
Abstract
An ink jet print head includes a silicon ink jet chip, a print
head holder, configured to carry and support the silicon chip, and
a glass plate, bonded between the silicon chip and the print head
holder. The ink jet chip has a coefficient of thermal expansion
.alpha..sub.s. The print head holder has a holder wall thickness,
and a coefficient of thermal expansion .alpha..sub.h that is
substantially different from .alpha..sub.s. The glass plate has a
coefficient of thermal expansion .alpha..sub.g that is
substantially similar to .alpha..sub.s, and a thickness at least as
great as the holder wall thickness, whereby stress created by
differential thermal expansion between the silicon chip and the
holder is attenuated by the glass plate.
Inventors: |
Karlinski; Haggai; (Ramat
Chen, IL) ; Fisher; Gil; (Shoham, IL) ;
Nathan; Roi; (Halfa, IL) ; Weiss; Ilan;
(Kfar-Saba, IL) |
Correspondence
Address: |
HEWLETT PACKARD COMPANY
P O BOX 272400, 3404 E. HARMONY ROAD, INTELLECTUAL PROPERTY ADMINISTRATION
FORT COLLINS
CO
80527-2400
US
|
Family ID: |
38692366 |
Appl. No.: |
11/447333 |
Filed: |
June 6, 2006 |
Current U.S.
Class: |
347/54 |
Current CPC
Class: |
Y10T 156/10 20150115;
B41J 2002/14362 20130101; B41J 2/14201 20130101 |
Class at
Publication: |
347/54 |
International
Class: |
B41J 2/04 20060101
B41J002/04 |
Claims
1. An ink jet print head, comprising: a silicon ink jet chip,
having a coefficient of thermal expansion .alpha..sub.s; a print
head holder, configured to carry and support the silicon chip,
having a holder wall thickness, and having a coefficient of thermal
expansion .alpha..sub.h that is substantially different from
.alpha..sub.s; and a glass plate, bonded between the silicon chip
and the print head holder, having a coefficient of thermal
expansion .alpha..sub.g that is substantially similar to
.alpha..sub.s, and a thickness at least as great as the holder wall
thickness, whereby stress created by differential thermal expansion
between the silicon chip and the holder is attenuated by the glass
plate.
2. An ink jet print head in accordance with claim 1, wherein the
thickness of the glass plate is greater than a thickness that is
proportional to a ratio of the modulus of elasticity of the glass
material to the modulus of elasticity of the material of the print
head holder.
3. An ink jet print head in accordance with claim 1, wherein the
thickness of the glass plate is about 2 times the holder wall
thickness.
4. An ink jet print head in accordance with claim 1, wherein the
holder wall thickness is no greater than 0.5 mm and the thickness
of the glass plate is at least 0.7 mm.
5. An ink jet print head in accordance with claim 1, wherein the
glass plate comprises a pair of glass plates of similar size and
shape, each glass plate being symmetrically bonded on a first side
to opposing sides of the silicon chip, and on a second side to the
holder.
6. An ink jet print head in accordance with claim 1, wherein
.alpha..sub.s and .alpha..sub.g are in the range of about
3.times.10.sup.-6/.degree. C., and .alpha..sub.h is in the range of
from 15 TO 17.times.10.sup.-6/.degree. C.
7. An ink jet print head in accordance with claim 1, further
comprising a glass membrane surrounding the ink jet chip, and
bonded between the ink jet chip and the glass plate.
8. An ink jet print head in accordance with claim 7, wherein the
glass membrane has a thickness about 1/10 the thickness of the
glass plate.
9. A silicon chip assembly, comprising: a silicon chip, having a
coefficient of thermal expansion .alpha..sub.s, and being
associated with heat-generating elements; a mounting structure,
configured to carry and support the silicon chip, having a wall
thickness, and having a coefficient of thermal expansion
.alpha..sub.h that is substantially different from .alpha..sub.s;
and a glass plate, bonded between the silicon chip and the mounting
structure, having a coefficient of thermal expansion .alpha..sub.g
that is substantially similar to .alpha..sub.s, and having a
thickness that is at least as great as the wall thickness, whereby
stress created by differential thermal expansion between the
silicon chip and the mounting structure is attenuated by the glass
plate.
10. A silicon chip assembly in accordance with claim 9, wherein the
thickness of the glass plate is about 2 times the wall thickness of
the mounting structure.
11. A silicon chip assembly in accordance with claim 9, wherein the
wall thickness of the mounting structure is less than about 0.5 mm
and the thickness of the glass plate is at least 0.5 mm.
12. A silicon chip assembly in accordance with claim 9, wherein
.alpha..sub.s and .alpha..sub.g are in the range of about
3.times.10.sup.-6/.degree. C., and .alpha..sub.h is in the range of
from 15.times.10.sup.-6/.degree. C. to 17.times.10.sup.-6/.degree.
C.
13. A silicon chip assembly in accordance with claim 9, further
comprising a glass membrane surrounding the silicon chip, and
bonded between the silicon chip and the glass plate.
14. A silicon chip assembly in accordance with claim 13, wherein
the glass membrane has a thickness about 1/10 the thickness of the
glass plate.
15. A silicon chip assembly in accordance with claim 9, wherein the
glass plate comprises a pair of glass plates of similar size and
shape, each glass plate being symmetrically bonded on a first side
to opposing sides of the silicon chip, and on a second side to the
mounting structure.
16. A silicon chip assembly in accordance with claim 9, wherein the
silicon chip comprises a micromachined ink jet chip, and the
mounting structure comprises a print head holder, having internal
ink passageways configured to provide liquid ink to the
micromachined ink jet chip.
17. A method for reducing stress between a silicon chip and a
bonded mounting structure having a coefficient of thermal expansion
substantially different from a coefficient of thermal expansion of
the silicon chip, comprising the step of: bonding a thermal
stress-attenuating layer between the silicon chip and the mounting
structure, the thermal stress-attenuating layer having a
coefficient of thermal expansion that is substantially similar to
the coefficient of thermal expansion of the silicon chip.
18. A method in accordance with claim 17, wherein the step of
bonding a thermal stress-attenuating layer between the silicon chip
and the mounting structure comprises bonding a pair of thermal
stress-attenuating layers symmetrically on opposing sides of the
silicon chip.
19. A method in accordance with claim 17, wherein the step of
bonding a thermal stress-attenuating layer comprises bonding a
thermal stress-attenuating layer having a thickness that is greater
than a wall thickness of the mounting structure.
20. A method in accordance with claim 17, wherein the step of
bonding a thermal stress-attenuating layer comprises bonding a
glass layer between the silicon chip and the mounting structure,
the glass layer having a thickness that is greater than a wall
thickness of the mounting structure.
Description
BACKGROUND
[0001] In print head manufacturing, chips with micro-machined
silicon arrays are often attached to plastic holders. The
micro-machined silicon plates are often covered by a thin and
flexible glass membrane. The silicon array structure is in fluid
communication with an ink reservoir, and includes multiple ink
passageways communicating with ejection nozzles and having
actuators (e.g. piezoelectric firing elements) that are selectively
actuable to pressurize the ink and eject drops of ink onto print
media. The silicon array structure is often adhesively bonded
directly to the holder or mount, which can be made from plastic,
composite, or other suitable material. In addition to serving as a
structural mount or support for the printhead silicon, the holder
frequently includes an ink reservoir and other components of the
printing system.
[0002] One challenge presented by these structures is that there is
a large difference in the coefficient of thermal expansion of
silicon or glass and that of plastics. Consequently, differential
thermal expansion of the silicon array and the plastic holder can
produce significant mechanical stress in the glass membrane and the
silicon plate. As a result of this stress the silicon array can
bend or warp, causing the inkjet nozzles to loose directionality,
or it can even crack, destroying the print head. This difference in
expansion can also complicate print head production processes that
involve the application of elevated temperature, and can complicate
print head operation, since large temperature differences cannot be
tolerated during operation.
[0003] While it is possible to construct a print head holder of a
material having a coefficient of thermal expansion similar to
silicon or glass, this is generally not economical or practical,
and would adversely affect the cost of the print head module.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Various features and advantages of the invention will be
apparent from the detailed description which follows, taken in
conjunction with the accompanying drawings, which together
illustrate, by way of example, features of the invention, and
wherein:
[0005] FIG. 1 is a front view of a print head module having a
micro-machined silicon array bonded to it;
[0006] FIG. 2 is a side cross-sectional view of the print head
module of FIG. 1;
[0007] FIG. 3 is a front cross-sectional view of one embodiment of
a print head module having a glass plate bonded to the
micromachined array; and
[0008] FIG. 4 is a side cross-sectional view of the print head
module of FIG. 3.
DETAILED DESCRIPTION
[0009] Reference will now be made to exemplary embodiments
illustrated in the drawings, and specific language will be used
herein to describe the same. It will nevertheless be understood
that no limitation of the scope of the invention is thereby
intended. Alterations and further modifications of the inventive
features illustrated herein, and additional applications of the
principles of the invention as illustrated herein, which would
occur to one skilled in the relevant art and having possession of
this disclosure, are to be considered within the scope of the
invention.
[0010] As noted above, chips with micro-machined silicon arrays are
often attached to plastic holders in print head arrays. Such a
configuration is depicted in FIGS. 1 and 2. The inkjet print head
module 10 includes an ink ejection structure 12 formed by a
micro-machined silicon plate 14 with a flexible and thin glass
membrane 16 bonded to it. The micro-machined silicon plate includes
plurality of ink channels formed in one or both of its side
surfaces 18, and a plurality of actuators 20 (e.g. piezo electric
actuators) are disposed adjacent to each of the ink channels for
pressurizing and ejecting ink droplets 22 onto print media 24 (e.g.
paper) disposed below the print head module 10.
[0011] The ink ejection structure 12 is bonded to a holder 26 by
means of adhesive, such as epoxy. The holder supports the ink
ejection structure and also includes ink inlets 28 that lead to an
internal ink reservoir 30 (shown in the cross-sectional view of
FIG. 2), which supplies and distributes ink to the ink channels and
nozzles of the ink ejection structure, allowing the ink to be drawn
in and ejected as described above. The holder can also include
registration pins 32 that provide a mechanical interface between
the micro-machined silicon array and a mechanical frame (not shown)
of the printer system.
[0012] The holder 26 can be made from plastic or polymer materials,
composite materials, or any other suitable material. As noted
above, however, there is a large difference in the coefficient of
thermal expansion of silicon or glass on the one hand, and that of
plastic or polymer materials. Specifically, silicon and glass each
have coefficients of thermal expansion that are around
3.times.10.sup.-6/.degree. C., while that of polymer materials
frequently used for print head modules is typically around
15-17.times.10.sup.-6/.degree. C.
[0013] It will be appreciated that the actuators 20 generate heat,
as do other parts of the printing system, and this heat is
naturally dispersed throughout the whole system. However, a given
change in temperature of the entire system will produce
differential expansion of the various components, depending upon
their respective coefficients of thermal expansion. Differential
expansion of the micromachined array 12 and the plastic holder 26
can produce significant mechanical stress in the glass membrane 16
and the silicon plate 14. As a result of this stress the
micromachined array can bend, affecting the directionality of the
inkjet nozzles. Even worse, the glass membrane or silicon chip can
crack, destroying the print head. The difference in thermal
expansion also complicates print head production, which includes
processes that involve the application of elevated temperature,
such as for curing adhesives or thermally sealing cavities.
Differential thermal expansion can also complicate normal print
head operation, since large temperature differences cannot be
tolerated. While it is possible to construct a print head holder of
a material having a coefficient of thermal expansion similar to
silicon or glass, this is generally not economical or practical,
and would adversely affect the cost of the print head module.
[0014] Advantageously, the inventors have developed a structure and
method that reduces the stress between a polymer mounting structure
and a silicon structure that is bonded thereto. While the structure
and method are disclosed herein as applied particularly to inkjet
print heads, including micro-machined print heads, it is not
limited to these. Rather, it relates generally to any structure
having a silicon chip or substrate that is bonded to plastic or
some other material having a significantly different coefficient of
thermal expansion.
[0015] One embodiment of a print head module 100 having an improved
configuration is shown in FIGS. 3 and 4. In this embodiment, the
print head module generally includes a holder body 102 of polymer
or other material, with a micro-machined silicon array ink ejection
structure 104 attached to it. Like the print head structure shown
in FIG. 1, the silicon array includes a micro-machined silicon
plate 106 with a flexible and thin glass membrane 108 bonded to it,
such as by anodic bonding or by adhesive, such as epoxy. The
thickness of the glass membrane can be in the range of about
50microns, though it is not limited to this thickness. Like the
embodiment of FIG. 1, the micro-machined silicon plate includes a
plurality of ink channels formed in one or both of its side
surfaces 110, and a plurality of actuators, such as piezo electric
actuators (not shown) for pressurizing and ejecting ink droplets
from each ink channel onto print media (not shown).
[0016] The holder body 102 includes ink inlets 112 that lead to an
internal ink reservoir 114, which provides ink to the silicon array
104. The holder body can also include slots 116 for receiving
registration pins to provide a mechanical interface between the
micro-machined silicon array and a mechanical frame (not shown) of
the printer system.
[0017] Unlike the embodiment of FIGS. 1 and 2, the silicon array
104 is not bonded directly to the holder 102. Instead, in the
embodiment of FIGS. 3 and 4, the silicon array is bonded (by, e.g.
epoxy or other adhesive) to a pair of relatively thick glass
mounting plates 118 that are disposed symmetrically on both sides
of the silicon array. That is, each side surface 110 of the array
is bonded to one side of each glass plate. The opposite side of
each glass plate is in turn bonded, e.g. by adhesive, such as
epoxy, to the plastic holder 102.
[0018] Glass has a thermal expansion coefficient that is nearly
identical to that of silicon. Specifically, as noted above, both
silicon and glass have coefficients of thermal expansion that are
around 3.times.10.sup.-6/.degree. C. However, the holder 102
expands at a rate that is significantly different from glass. For
example, polymer materials frequently used for print head modules
have a coefficient of thermal expansion in the range of 15 to
17.times.10.sup.-6/.degree. C.
[0019] Advantageously, the thickness of the glass mounting plates
118 enables these plates to absorb and attenuate the resultant
mechanical stress caused by differential thermal expansion of the
silicon array 104 and the holder body 102. The thickness of glass
plates is selected such that it enables absorption (attenuation) of
forces introduced by thermal expansion of the plastic holder, and
does not transfer stress induced by the elevated temperature to the
fragile silicon chip ink jet array. Several factors contribute to
this function. First, the glass mounting plates are attached to a
relatively thin wall section 120 of the holder. The glass mounting
plates have a thickness that is at least as great as that of the
thin wall section of the holder to which they are bonded. More
broadly, the glass plates can have a thickness that is from about 1
to 3 times as thick as the holder wall thickness.
[0020] As used herein, the term "holder wall thickness" refers to
the minimum typical thickness of the wall 120 of the holder 102 in
the region where the glass plates 118 are bonded. While the holder
can include gussets and other thicker reinforcing structures that
connect to the holder wall and may be integrally formed with it
(e.g. by injection molding) in this region, it is the minimum
typical wall thickness in this region that is of interest. The
holder wall thickness typically varies from about 0.3 mm to about
0.5 mm. Accordingly, the glass plate thickness can range from about
0.3 mm to about 1.5 mm. In one specific embodiment, the glass
mounting plates have a thickness of about 0.7 mm, and the holder
wall thickness adjacent thereto is about 0.5 mm. The amount of
force produced by a particular structure under a given amount of
thermal expansion is smaller for a smaller structure. Thus, a
thinner holder wall will produce a smaller expansive force than
would a thicker wall, and a comparatively thicker
stress-attenuation layer will provide a greater force to resist
that expansive force.
[0021] The thickness of the glass plates 118 also relates to the
modulus of elasticity (Young's modulus) of glass versus that of the
polymer material of the holder. Polymer materials typically have a
modulus of elasticity in the range of from less than 1 to about 4
GPa. Glass, on the other hand, has a modulus of elasticity in the
range of about 64 Gpa. Thus a glass plate having the same overall
stiffness as the plastic holder would have a thickness that is less
than the holder wall thickness. (In order to have the same
stiffness as the holder, the glass plate thickness would be
proportional to the ratio of the modulus of elasticity of the glass
and that of the plastic holder material.) Consequently, where the
glass plate has a thickness of from 1 to 3 times that of the holder
wall thickness, the mechanical strength of the glass plate and its
ability to absorb mechanical stress will be substantially greater
than that of the holder wall. To adequately absorb the stress
caused by differential thermal expansion, a more elastic (i.e.
having a lower modulus of elasticity) stress-attenuation layer will
need to be thicker, while a more rigid (i.e. having a higher
modulus of elasticity) one can be thinner and still adequately
absorb the stress.
[0022] Additionally, the thickness of the glass plates reduces the
stress produced by differential expansion because stress is a
function of force and cross-sectional area of a material. Where
there is more material to absorb a given force, the resultant
stress will be lower. Since the glass is thicker than the plastic
walls of the holder, it makes the silicon array structure stiffer,
enables isolation of forces introduced by the plastic expansion
(due to elevated temperatures) and protects the fragile silicon
chip structure. This reduces the number of print head failures,
chip cracks, and increases production yield.
[0023] The glass plates 118, used as a stress-attenuation or
stress-absorption membrane, interface both with the silicon array
chip 104 and the plastic housing 102. The greater thickness of the
glass plates 118 absorbs the stress produced by differential
thermal expansion of the holder 102, and does not transfer this
stress to the fragile silicon chip array 104. Additionally, the
glass mounting plates stiffen the print head module as a whole, and
make it less sensitive to changes in temperature that occur during
bonding or in the course of print head use.
[0024] While the disclosure depicts an embodiment of a print head
module, the principles disclosed herein apply to any structure
wherein a silicon structure is bonded to plastic or some other
material having a significantly different coefficient of thermal
expansion. Accordingly, there is provided a system and method for
attenuating stress from differential thermal expansion between
silicon chips/devices and a bonded mounting structure, and in
particular, such a system for an inkjet print head structure.
[0025] It is to be understood that the above-referenced
arrangements are illustrative of the application of the principles
of the present invention. It will be apparent to those of ordinary
skill in the art that numerous modifications can be made without
departing from the principles and concepts of the invention as set
forth in the claims.
* * * * *